1. Field of Invention
The present invention relates to a capacitor, more specifically, a symmetrical capacitor.
2. Related Art
FIG. 1 illustrates a prior art capacitor structure; FIG. 2 is a cross-sectional view of the capacitor. The prior art capacitor includes an upper electrode plate 11 and a lower electrode plate 12, wherein the upper electrode plate 11 is connected to a wire 13 and the lower plate 12 is connected to a wire 15 through via hole 14. The upper electrode plate 11 and the lower electrode plate 12 are located on the two surfaces of the dielectric layer 16 respectively.
In the design of conventional MIM series capacitors, the capacitors are located horizontally as shown in FIG. 2. When stacking them vertically, one side must be output vertically, therefore the capacitor structure will be asymmetrical.
When this type of capacitor operates at low frequencies, the S-parameters of passive elements are generally reversible, that is, the output port and the input port have the same electrical features. However, when this type of capacitor operates at high frequencies, their asymmetrical structure would lead to different reflection parameters at different output ports. Therefore, S11 of the S-parameters is not equal to S22 at high frequencies. This type of capacitor is disclosed in U.S. Pat. No. 5,400,210 and U.S. Pat. No. 5,161,086, both of which concern embedded capacitors with an asymmetric multi-layer structure.
In order to change the asymmetric structure and to decrease the number of via holes, the prior art discloses a comb-shaped capacitor structure as shown in FIG. 3. As shown in the drawing, the capacitor includes two comb-shaped electrodes 21 and 22 that are connected to wires 23 and 24 respectively. This type of capacitor solves the asymmetric structure problem to a certain degree and reduces the number of via holes, therefore it has better high-frequency effects. However, because of its plane coupling structure, this type of capacitor has a low capacitance and occupies a relatively large area.